Glenn-
I don't know if you are looking for a diy project, but...
If you just want to play bass, a good option would be something like the Korg PX3B, which I used when I was learning bass a number of years ago. It sounds like a real bass (not a bass played through a hifi headphone amp) and has effects, tuner, metronome, etc etc built in as well. It's the size of a cassette tape box. It also has an AUX IN for playing along with recorded music.

I think Korg has a newer version available these days.

If you decide you want a PX3B, drop me an email/pm. I don't use mine anymore- actually I never used it enough to get any good on bass- and would be willing to sell it or trade for some audio/electronics parts.
Cheap.

A lot of the multi-effects units (for guitar as well as bass) have an aux in- you just have to check the operating manual- which you can generally download before you make a decision to purchase.
I've had a couple of Korg units and they seemed good value- and reasonably quiet as well.

HERE is one that I designed based on an LM386 chip. It works very well and has a low parts count. You don't have to include the distortion part if you don't want to. Volume is controlled by your bass. It doesn't have an ipod input, but I think it could be added fairly easily.